Field of the Invention
This invention relates to a process for forming a zinc-containing phosphate conversion coating layer on an active metal surface, more particularly a surface selected from the group consisting of (i) steel and other non-passivating ferrous alloys that contain at least 50% by weight of iron, (ii) galvanized steel, (iii) other surfaces of zinc or its alloys that contain at least 50% by weight of zinc; and (iv) aluminum and its alloys containing at least 50% by weight of aluminum.
It is well known that zinc phosphate conversion coatings, particularly those of the modem xe2x80x9clow zincxe2x80x9d type, are capable of producing excellent corrosion-protective under-coatings for subsequent painting. It has been generally regarded in the prior art that two of the important characteristics of a xe2x80x9clow zincxe2x80x9d phosphating liquid composition are a phosphate concentration of at least 5 grams per liter of composition, this unit of concentration being hereinafter usually abbreviated as xe2x80x9cg/lxe2x80x9d, more preferably at least 10 g/l, and a weight ratio of phosphate to zinc concentrations that is at least 10:1. Processes that use such phosphating compositions produce a substantial volume of effluent water containing phosphate, which in most jurisdictions is a pollutant that must be abated. An object of this invention is to provide phosphating processes that utilize compositions with lower contents of pollutants but still achieve satisfactory corrosion resistance as undercoats for paint.
Except in the claims and the operating examples, or where otherwise expressly indicated to the contrary, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word xe2x80x9caboutxe2x80x9d in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description and claims, unless expressly stated to the contrary: percent, xe2x80x9cparts ofxe2x80x9d, and ratio values are by weight; the term xe2x80x9cpolymerxe2x80x9d includes xe2x80x9coligomerxe2x80x9d, xe2x80x9ccopolymerxe2x80x9d, xe2x80x9cterpolymerxe2x80x9d, and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; specification of materials in ionic form implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole, and any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to the objects of the invention; the term xe2x80x9cpaintxe2x80x9d and its grammatical variations includes any more specialized types of protective exterior coatings that are also known as, for example, lacquer, electropaint, shellac, top coat, base coat, color coat, and the like; and the term xe2x80x9cmolexe2x80x9d and its variations may be applied to ionic, chemically unstable neutral, or any other chemical species, whether actual or hypothetical, that is specified by the type(s) of atoms present and the number of each type of atom included in the unit defined, as well as to substances with well defined neutral molecules.
It has been found that with proper control of other characteristics of the phosphating composition and process, fully satisfactory underpaint corrosion resistance can be obtained from conversion coatings formed by a phosphating composition with smaller concentrations of phosphate and ratios of phosphate to zinc than has heretofore been taught. The pollution potential of the phosphating compositions is correspondingly reduced. On some substrates, the corrosion resistance is actually improved over that obtained with current conventional processes using otherwise similar phosphating compositions with higher concentrations of phosphate
A working phosphating composition according to the invention comprises, preferably consists essentially of, or more preferably consists of water and the following components:
(A) a component of dissolved phosphate anions that have a concentration in the working composition that is not more than, with increasing preference in the order given, 13, 10, 9.5, 9.0, 8.5, 8.0, 7.5, 7.0, 6.5, and at least for economy still more preferably is not more than 6.0, 5.5, 5.0, 4.5, 4.0, 3.5, 3.0, or 2.5 g/l and independently preferably is at least, with increasing preference in the order given, 0.5, 1.0, 1.5, 1.7, 1.9, or 2.0 g/l;
(B) a component of dissolved zinc cations that have a concentration in the working composition that is at least, with increasing preference in the order given, 0.30, 0.40, 0.50, 0.60, 0.70, 0.75, 0.80, 0.85, 0.90, 0.93, 0.95, or 0.97 g/l and independently preferably is not more than, with increasing preference in the order given, 2.0,1.8, 1.6, 1.50, 1.40, 1.35, 1.30, 1.25, 1.20, 1.15, 1.10, 1.05, or 1.00 g/l; and
(C) a component of dissolved manganese(II) cations that have a concentration in the working composition that is at least, with increasing preference in the order given, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, or 0.48 and independently preferably is not more than, with increasing preference in the order given, 2.0, 1.5, 1.2, 1.0, 0.90, 0.80, 0.75, 0.70, 0.65, 0.60, 0.55, or 0.50;
and, optionally, one or more of the following components:
(D) a component of at least one of:
dissolved nickel(II) cations that have a concentration in the working composition that is at least, with increasing preference in the order given, 0.10, 0.20, 0.30, 0.40, 0.50, or 0.60 g/l and independently preferably is not more than, with increasing preference in the order given, 2.0,1.5,1.3, or 1.1 g/l; and
dissolved copper cations that have a concentration that is at least, with increasing preference in the order given, 0.0001, 0.0003, 0.0005, 0.0007, 0.0009, 0.0011, 0.0013, 0.0015, 0.0017, 0.0019, or 0.0021 g/l and independently preferably is not more than, with increasing preference in the order given, 0.030, 0.025, 0.020, 0.015, 0.010, or 0.070 g/l;
(E) a component of dissolved fluorine-containing anions that have a stoichiometric equivalent as fluoride that is at least, with increasing preference in the order given, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, or 0.55 g/l and independently preferably is not more than, with increasing preference in the order given, 2.0, 1.5, 1.2, 1.0, 0.90, or 0.80 g/l;
(F) a component of dissolved nitrate ions that have a concentration in the working composition that is at least, with increasing preference in the order given, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, or 3.9 g/l and independently, at least for economy, preferably is not more than, with increasing preference in the order given, 20, 15, 12, 10, or 8.8 g/l; and
(G) a dissolved accelerator component consisting of at least one substance selected from the group consisting of:
0.3 to 4 g/l of chlorate ions;
0.01 to 0.2 g/l of nitrite ions;
0.05 to 2 g/l of m-nitrobenzene sulphonate ions;
0.05 to 2 g/l of m-nitrobenzoate ions;
0.05 to 2 g/l of p-nitrophenol;
0.005 to 0.15 g/l of hydrogen peroxide in free or bound form;
0.1 to 10 g/l of hydroxylamine in free or bound form; and
0.1 to 10 g/l of reducing sugar.
If the composition has an initial pH value lower than 3.80+0.03, it has positive Free Acid points which are quantitatively defined as equal to the number of milliliters (hereinafter usually abbreviated as xe2x80x9cmlxe2x80x9d) of 0.100 N strong alkali required to titrate a 10.0 ml sample of the composition to a pH value of 3.80xc2x10.03; if the initial value of pH of the composition is higher than 3.80xc2x10.03, it has negative Free Acid points, which are defined as the negative number with the same absolute value as the number of ml of strong acid required to titrate a 10 ml sample of the composition to a pH of 3.80+0.03. If the initial composition has a pH of 3.80xc2x10.03, it has 0.0 points of Free Acid. In addition to containing the above-noted components, a working composition according to the invention preferably has a Free Acid value that is at least, with increasing preference in the order given, xe2x88x921.0, xe2x88x920.5, 0.0, 0.10, 0.20, 0.30, 0.40, or 0.49 points and independently preferably is not more than, with increasing preference in the order given, 3.0, 2.5, 2.0, 1.90, 1.80, 1.70, 1.60, 1.50, 1.40, 1.30, 1.20, or 1.11 points.
The presence of nickel cations in a composition according to the invention is preferred, unless the anti-pollution laws in the jurisdiction where the composition is used make the presence of nickel impractical economically. In such an instance, the presence of copper cations is alternatively preferred, unless they too are economically impractical because of pollution.
The presence of fluoride containing anions in a composition according to the invention is generally preferred, especially when phosphating aluminum under most conditions. When phosphating steel or zinciferous surfaces such as galvanized steel, all of the fluoride present is preferably complex fluoride, but when phosphating aluminum, some of the fluoride is preferably present as xe2x80x9cfree fluoridexe2x80x9d, a characteristic of the composition that can be measured by a fluoride ion sensitive electrode in contact with the composition and electrically connected to a reference electrode also in the same volume of composition, as known to those skilled in the art. Complex fluoride is preferably supplied to a composition according to the invention by at least one of tetrafluoroboric acid, hexafluorosilicic acid, hexafluorotitanic acid, hexafluorozirconic acid, and salts of all of these acids. At least for economy, hexafluorosilicic acid is most preferred. When free fluoride is needed or desired, it is preferably supplied by hydrofluoric acid and/or ammonium hydrogen fluoride.
The presence of nitrate in a composition according to the invention is preferred, and independently the nitrate is preferably provided at least in part by nitric acid, although nitrate salts may also be used. When nitrate is used, it preferably is present in a ratio to phosphate that is at least, with increasing preference in the order given, 0.20:1.00, 0.25:1.00, 0.30:1.00, 0.37:1.00, 0.39:1.00, 0.41:1.00, 0.80:1.00, 1.2:1.00, 1.6:1.00, or 1.9:1.00 and independently, at least for economy, preferably is not more than, with increasing preference in the order given, 30:1.00, 20:1.00,10:1.00, 5:1.00, 3.0:1.00, 2.5:1.00, 2.2:1.00, or 2.0:1.00. The major identified reason for a preference for the presence of nitrate in at least the above ratios to phosphate is an improved resistance to corrosion after painting in such tests as GM 9540P, particularly on cold rolled steel.
If only zinciferous surfaces are to be phosphated, an accelerator component is not needed in a composition according to the invention, but for predominantly ferriferous and/or aluminiferous surfaces an accelerator is preferred. If there is no objection from an operator of a phosphating process to monitoring the concentration of accelerator and replenishing it as needed from a source that is distinct from the source of other replenishing ingredients, nitrite is generally preferred as the accelerator, because of its high technical reliability and effectiveness at a low concentration. When nitrite is used as the accelerator, its concentration preferably is at least, with increasing preference in the order given, 0.03, 0.05, 0.07, 0.09, or 0.11 g/l and independently preferably is not more than 0.18, 0.16, 0.14, or 0.12 g/l. If nitrite is considered hazardous because of the possibility of generation of nitrous oxides or other noxious materials from its misuse, similar advantages may be obtained by the use of hydrogen peroxide. (Because both nitrite and hydrogen peroxide are subject to fairly rapid decomposition in acid solutions, they preferably are not added to a phosphating composition until shortly before it begins to be used and therefore preferably are not included in make-up or replenisher concentrates.)
If the convenience of a single package replenisher is preferred, hydroxylamine in one of its stable bound forms is preferred as the accelerator. Salts of hydroxylamine with any strong acid are generally stable enough in compositions according to the invention to be practically included in single package concentrates, with the sulfate being particularly preferred at least for economy. Oximes can also serve as a suitable source of hydroxylamine. Irrespective of the specific source, when hydroxylamine is used as the accelerator in a working composition according to this invention, the concentration, measured as its stoichiometric equivalent as hydroxylamine, preferably is at least, with increasing preference in the order given, 0.20, 0.25, 0.30, 0.33, 0.36, or 0.39 g/l and independently preferably is not more than, with increasing preference in the order given, 1.5, 1.0, 0.90, 0.80, 0.85, 0.80, 0.75, 0.70, 0.65, or 0.61 g/l.
A phosphating process according to the invention can be accomplished by contacting a suitably prepared substrate with a composition according to the invention. Any method of achieving contact may be used, with one of immersion and spraying generally being preferred, depending on the size and the complexity of the shape of the surface to be phosphated, as generally known in the art. Consistent phosphating results are generally obtained when, and it is therefore preferred that, the temperature of the phosphating composition is controlled while it is in contact with the surface being phosphated. This temperature preferably is at least, with increasing preference in the order given, 30, 35, 37, 39, 41, or 43xc2x0 C. and independently, primarily for economy, preferably is not more than, with increasing preference in the order given, 85, 75, 70, 65, 63, 61, 59, 57, or 55xc2x0 C.
The mass of the phosphate coating formed can be determined by methods known in the art. This characteristic of a process according to the invention is generally reported as xe2x80x9ccoating weightxe2x80x9d, which is defined as the mass of the coating in grams divided by the surface area of the coating in square meters (hereinafter usually abbreviated as xe2x80x9cg/m2xe2x80x9d). For predominantly ferriferous surfaces such as cold rolled steel, the coating weight preferably is at least, with increasing preference in the order given, 0.50, 0.60, 0.70, 0.80, or 0.86 g/m2 and independently preferably is not more than, with increasing preference in the order given, 5.0, 4.5, 4.0, 3.5, 3.3, 3.0, 2.8, or 2.6 g/m2. For predominantly zinciferous surfaces such as all types of galvanized steel, the coating weight preferably is at least, with increasing preference in the order given, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, or 1.10 g/m2 and independently preferably is not more than, with increasing preference in the order given, 7.0, 6.5, 6.0, 5.5, 5.0, 4.5, 4.1, or 3.8 g/m2. For predominantly aluminiferous surfaces such as commercial aluminum alloys, the coating weight preferably is at least, with increasing preference in the order given, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, or 1.05 g/m2 and independently preferably is not more than, with increasing preference in the order given, 5.5, 5.0, 4.5, 4.0, 3.5, 3.0, 2.8, 2.6, 2.4, or 2.2 g/m2. (All of these coating weight preferences are based on corrosion test results and may need to be changed in special circumstances.)
The time of contact between the phosphating composition and the substrate in a process according to the invention is generally not at all critical if the desired coating weight is achieved, presumably because the rate of formation of the coating is much faster at the beginning of contact of a fresh metal surface with a phosphating composition than after even a thin phosphate coating has initially formed. As a general guideline, when contact is by immersion, the contact time preferably is at least, with increasing preference in the order given, 0.2, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, or 1.9 minutes and independently preferably is not more than, with increasing preference in the order given, 30, 20, 15, 10, 5, 3.0, 2.7, 2.5, 2.3, or 2.1 minutes; and when contact is by spraying, the contact time preferably is at least, with increasing preference in the order given, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, or 0.95 minutes and independently preferably is not more than, with increasing preference in the order given, 10, 7, 5, 4.0, 3.5, 3.0, 2.5, or 2.1 minutes.
Before being contacted with a composition according to the invention, a substrate to be phosphated in a process according to the invention is preferably cleaned, rinsed, and activated by any of the means known for these purposes in the art. Some preferred, but by no means exclusive, embodiments are illustrated in the examples below. Similarly, after the desired time of contact between a phosphating composition according to the invention and a substrate has been completed, the substrate is preferably removed from contact with any phosphating composition, rinsed with water, and optionally further treated as known in the art and illustrated in the examples below.